1. Field of the Invention
This invention relates generally to capacitive deionization (CDI) of liquids containing charged species, such as industrial waste streams, city sewage and seawater, etc. More particularly, this invention relates to replaceable flow-through capacitors (FTCs) for removing charge-carrying materials from inorganic and organic aqueous solutions.
Water is one of the most precious resources to human. It is appropriate to describe the importance of water as xe2x80x9cwater gives us lifexe2x80x9d. About three-quarter of the earth surface is covered by water, 98% of which is salt water and the remaining 2% is fresh water. Most of the fresh water is trapped in polar ice caps leaving less than 1% be suitable for direct use. As population grows with increasing urbanization and industrialization. which augment water consumption and waste generation, many countries in the world are thus suffering from water scarcity and environmental pollution. According to a UN prediction, there will be 48 countries, accounting for 32% of the world population, lack of fresh water by the year 2025. Water is vital to both the quality of life and prosperity of economy. Contaminated water can impart illness to people and cost governments millions of dollars to clean the polluted sites. Without fresh water, the daily life and activities of human will be hampered. Providing sufficient sources of fresh water to their people may be the responsibility of governments, yet it is the obligation of people on this globe to conserve water use, to recover useful resources and to reduce pollution.
Desalination of seawater is likely the most economical way to produce fresh water. Seawater is free, unlimited and available all year long unaffected by weathers. Seawater also has the highest content of total dissolved solids (TDS) among all brackish waters. Furthermore, liquid wastes, whether organic or inorganic in nature, frequently contain charged species resulted from hydrolysis, decomposition, flocculation, biological or chemical reactions of solutes. The total amount of charged species in seawater and liquids are expressed as TDS in terms of ppm (parts per million). Whether in desalination or waste treatments, reduction of TDS or deionization is one of the major goals of a purification protocol which may include sedimentation, adsorption, filtration, ozonation, etc. The aforementioned processes are arranged at either upstream or downstream of deionization for pre-treating or post-treating feed liquids. TDS of feed liquids has to be reduced to a certain level of ppm for domestic consumption or industrial applications. Deionization may be achieved using some popular techniques such as ion-exchange, distillation, reverse osmosis (RO), and electrodialysis. In comparison, CDI is relatively new and less known to the general public. In choosing a deionization method, one has to consider the cost of materials and operation, permeate flux (or yield , as well as salt rejection rate.
Most ion-exchange resins for deionization are expensive synthesized materials, which will release relatively benign ions in exchange for toxic ions. With use, the resins will become saturated and require regeneration. Regeneration of ion-exchange resins demands the use of strong acids or strong bases, as well as a lot of rinsing water. Hence, ion-exchange is water-wasteful, and it generates secondary pollution from chemicals used for regeneration. In order to attain fresh water, heat must be invested to seawater or raw waters in a distillation pot. Ions are then left behind as sludge in the pot. Distillation is an energy intensive process requiring a large operation space, for example, an evaporation column of more than 100 m ( greater than 300 ft) tall is employed as taught in U.S. Pat. No. 4,636,283 issued to Nasser. Nevertheless, distillation is the most widely used method in the desalination of seawater in the world. Commercially, RO is the second most popular desalination technique. It uses membranes to extract fresh water from brackish waters, while ions are still present in the feed waters. RO depends on high pressure, e.g., 800-1200 psig, to force the permeate (pure water) to pass through the nano-pores of the membranes. High operation pressure and low permeate flux are two disadvantageous characters of RO. While a high operation pressure of RO means a high operation cost, a low permeate flux imparts low throughputs. Lastly, electrodialysis utilizes both a DC electric field and ion-permeable membranes for deionization. There are at least three compartments, i.e., anode, middle and cathode, in each electrodialysis unit. Brackish waters flow in the middle compartment, and ions are drawn to the anode or the cathode compartment by electrostatic attraction in conjunction with selective permeation of membranes. Both the electrodes and the membranes of electrodialysis are extremely costly for treating large quantity of liquids. In terms of pollution reduction, ions are not entrapped or collected in the membrane techniques such as RO and electrodialysis. Therefore, these techniques are merely for liquid purification methods rather than for pollution reduction. On the contrary, CDI is a technique that can perform purification and pollution reduction with high recycle rates of feed liquids.
While solid particles are easily removed by a filtering media, charged species can be effectively detained by an electric field. It is a natural phenomenon that ions respond swiftly and reversibly to an electrostatic attraction. Within liquids containing charged species, such electrostatic attraction is most conveniently created between two parallel conductive plates under an application of an electric current. As charged species flow through the charged plates, they are quickly adsorbed by the plates, resulting in the reduction of TDS. Such a method of desalination is named as capacitive deionization (CDI), and the setup for carrying CDI is known as flow-through capacitor (FTC). The application of FTC was published three decades ago by J. Newman et. al., in J. Electrochem Soc.: Electrochemical Technology, March 1971, Pages 510-517, entitled xe2x80x9cDesalting by Means of Porous Carbon Electrodesxe2x80x9d. It is incorporated by reference herein. Three representative prior arts of CDI are cited here to examine their commercial perspective, they are U.S. Pat. No. 5,779,891 issued to Andelman, U.S. Pat. No. 5,858,199 to Hanak and U.S. Pat. No. 5,954,937 to Farmer. All three patents are also incorporated by reference herein. The ""891 patent uses graphite foil as conductive backing and activated carbon as ion-adsorbing material to form electrodes by physical compression. No chemical bonding, which is more reliable in electric connection than physical compression, exists between the active material and the substrate. A gold compression contact is also used to bind the electrode leads and the wire leads, which are connected to a DC power supply. The inclusion of precious metal will economically prevent the FTC from becoming a consumer product. Furthermore, platinum or other catalytic metal is doped with the carbon electrodes in ""891 patent for electrochemically destroying chlorinated hydrocarbons (CHCs). For a higher efficiency of detoxification, CHCs are better treated with ozone than electrolysis. In order to attain the shortest and the straightest flow path, holes arc cut on the electrodes of FTC in ""891 patent for aligning with the perforation of the liquid-feeding pipe. Not only the adsorbing area is compromised, the specific orientation of the electrodes also complicates the manufacturing process, which again will increase the cost of the devices and impair the commercial merit of the product.
For the separation of ionic substances from liquids, the patent ""199 utilizes a combination of an electric field and a compound centrifugal force or Coriolis force. Incorporating a mechanical rotator in the apparatus results in a high operation cost and is difficult to use. In addition to carbons, the ""199 patent also proclaims the use of mixed metal oxides such as tantalum and ruthenium oxides as the active material for ions adsorption. The oxides are expensive and not suitable for treating a large quantity of liquids. An expensive, synthesized aerogel carbon is employed as the ion-adsorbing material in the ""937 patent for desalting. Such a carbon material is prepared from the polycondensation of resorcinol with formaldehyde followed by super critical point drying in liquid CO2 under a pressure as high as 900 psi, and a carbonization of the dried gel at 500-1200xc2x0 C. The preparation of a single batch takes as long as 2 weeks to complete according to U.S. Pat. No. 4,997,904 issued to Pekala, which is incorporated by reference herein. The lengthy preparation in conjunction with high processing pressures and temperatures will impart high costs to the materials required for constructing the FTC. Furthermore, the ""937 patent uses gaskets, bolts, nuts, end plates and multi pairs of electrode to assemble a multi-stage FTC. Such a complex arrangement of FTC is prone to leaks and lacks commercial viability. CDI is truly a viable technology for reducing the TDS of liquids. However, more research is needed identify economical materials for use in CDI and to identify FTC devices that provide n easy mass-production and a user-friendly operation for the technique to become an affordable commercial means for recycling used liquids and waste reduction, as well as or generating fresh water from seawater.
Removal of ions by CDI utilizes a common mechanism as energy storage in supercapacitors, (i.e. supercapacitors can store up to several thousands of farad (F) of charges). Both CDI and supercapacitors mainly depend on double layer capacitance (DLC), as first characterized by Helmholtz in 1879, for deionization or for energy storage. According to the instant invention, nanoparticles including ferrites, such as hydrated iron compound FexOyHz, where 1.0xe2x89xa6xxe2x89xa63.0, 0.0xe2x89xa6yxe2x89xa64.0, and 0.0xe2x89xa6zxe2x89xa61.03, and the main component of the particles is the black magnetite (Fe3O4) are first synthesized. Thereafter, by means of roller coating, powder coating or electrophoretic deposition, the said nanoparticles are secured to suitable current collectors to form conformal, monolithic electrodes of FTC for waste treatments and desalination. Preparation of the hydrated powders uses inexpensive starting materials, and the manufacturing procedures are quick and simple. The ion-adsorbing materials of the present invention are thus economical.
FTC of the instant invention is constructed from concentrically winding two sheets of electrodes interposed with polymeric dividers into a hollow-center roll. The central opening provides an insertion of a perforated pipe for feeding liquids to the FTC, while the dividers perform three tasks: 1) providing an insulation to the electrodes, 2) confining fluids within the FTC, and 3) creating a transverse flow of fluids inside the capacitor. Under the driving force of a peristatic pump, liquids flow freely, horizontally and outwardly through the entire length of electrodes before leaving the capacitor. Ions, thereby, have the highest opportunity to contact all accessible surface areas of the electrodes for adsorption. Not only the quantity of active materials used to fabricate the electrodes is minimum, both fabrications of the electrodes and the capacitors can be easily automated. The FTCs production hence becomes affordable consumer products for liquid purification and desalination. As known to those skilled in the art, the capacitors can be completely regenerated under a flushing with a clean solvent coupled with a quick reversion of the polarity of electrodes. Moreover, the FTC of the present invention is inexpensive; thus, it is dispensable. The capacitor can also be replaced and disposed when regeneration appears ineffective in restoring the deionization capability. Both the ion-adsorbing materials and the current collector of the replaceable FTC of the present invention are recyclable and environment friendly.
Just like the commercial packages of adsorbing/filtering carbons, ion-exchange resins and RO membranes, which are normally in a compact form for easy replacement, the FTC of the present invention is designed in a free-standing configuration. Such a FTC can be placed in a housing holder equipped with input and output ports for liquids, two electric leads for connection to a power source, and a cap with gaskets for hermetic sealing. All components of the housing holder can be precisely fabricated by injection molding. Replacement of a used FTC with a new one is as easy as changing battery for a flashlight. As soon as the FTC is placed in the housing holder, a liquid feeding pipe is inserted to the center of FTC roll, electrode leads are connected to the electric pins atop the holder cap using snap-on connectors, then the holder is sealed by hand tightening the threaded cap. It takes less than one minute to complete the restoration of a CDI treating unit without using any tool. CDI can be operated by a DC power at as low as 0.5 Volt and 100 mA, which can be easily provided by batteries, solar cells and fuel cells. Comparing to distillation, RO and electrodialysis, power consumption of CDI is much lower. Low DC voltages are needed only to retain charged species on the electrode surface, whereas chemical reactions or electrolysis should be avoided for they are detrimental to energy efficiency and the lifetime of electrodes. Electricity is used instead of chemicals for regenerating the electrodes of FTC, therefore no secondary pollution is produced. Nevertheless, precious ions such as Au3+ and Pd2+ can be collected, concentrated, and converted to useful forms using the CDI technique.
Being compact, light and battery-operational, CDI is truly a mobile desalting technique for liquid purification and desalination. In addition to easy incorporation of CDI with an existing waste-liquids treatment system, the instant invention further offers economical ion-adsorbing materials and user-friendly FTC package.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.